Many industries utilize wet grinding and dispersion equipment for size reduction in dispersion processes to prepare very find dispersions of solid particles suspended in liquid medium. This wet grinding process is commonly used for photographic, pharmaceutical, cosmetic, electrical and other applications requiring fine particle formation. The processing chamber of these devices are commonly filled with small grinding beads which occupy up to eighty or ninety percent of the volume within the chamber. The agitation of these beads in the presence of the liquid dispersion containing the solid particles results in ultra-fine size reduction. The large number of contact points provided by the grinding beads provides an efficient way to impart energy into the dispersion or particles to be ground.
These continuous grinding mills are fitted with retention screens on the product outlet of the grinding chamber to retain the grinding beads and oversized product in the grinding zone and allow the liquid dispersion to flow continuously through this active grinding zone. The cylindrical screen cartridges are commonly used for fine grinding media and are useful on larger production mills. However, the screen openings of the cylindrical screen cartridges are prone to plugging and require frequent cleaning during the wet grinding process. In addition, dispersions exhibiting pseudo-plastic flow behaviors can build up a high resistance to flow and result in excessive pressures in the grinding chamber. Such screens are prone to plugging by large undispersed particles in the liquid phase and fine beads or bead fragments. Screen plugging results in reduced run times, process variability and reduced process reliability. In addition, higher production costs are associated with screen clogging. To use grinding beads in these mills requires very narrow size distribution of the beads and requires intensive bead sieving and bead preconditioning to remove any residual fines. In most cases additional bead preconditioning is required to remove beads that may fragment during the milling process. These processing steps increase the unit costs of manufacturing dispersed systems.
The present invention provides an advantage over the prior art by extending the commercially useful process latitude of existing bead milling technology to operate continuously to manufacture dispersed systems.